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peridotites. If a similar discrepancy exists for isotopes of other elements, and if such highly depleted domains constitute a signifi cant mass fraction of the mantle, then our current estimates, derived mostly from the isotopic composition of MORB, may underestimate the extent of depletion of the Earth’s mantle. Th is could have important implications for the evolution, size and formation age of the depleted mantle3, but could be a property unique to Os isotopes. In contrast to Os, isotopes of elements such as strontium, neodymium, hafnium and lead are more prone to perturbation by melt-rock reaction processes, which could eff ectively erase or mitigate their previous depletion record.

If, as suggested by Liu and colleagues,

the wide range of Os isotope ratios in the Arctic peridotites simply documents the presence of variably depleted ancient peridotites in the Earth’s mantle, then these rocks must have escaped homogenization by convective stirring with other more enriched materials (for example, recycled oceanic crust; Fig. 1) for billions of years. Th is apparent ineffi ciency of stirring in the Earth’s mantle is in good agreement with recent geodynamic models4.

Os isotope ratios similar to the two lowest reported by Liu and colleagues have until now been considered unique to the mantle attached to ancient continental regions — the cratonic subcontinental lithospheric mantle5 (Fig. 1). On the basis of the apparent lack of subcontinental lithospheric mantle signatures in the

Gakkel ridge MORB, Liu and colleagues argue that the Gakkel ridge peridotites must represent ancient depleted oceanic mantle, rather than continental mantle. If this is correct, the apparent similarity in Os isotope compositions of the subcontinental lithospheric mantle (SCLM) and oceanic mantle invalidates the use of highly depleted Os isotope ratios as unique tracers of the SCLM6. However, there are no other isotopic or chemical signatures that unambiguously distinguish between depletion originating in the SLCM and the oceanic mantle5. If highly depleted Os isotope signatures are not unique to the SCLM, and if, as suggested by Liu and colleagues, the depleted SCLM would be too refractory to melt underneath Gakkel ridge, then how could the SCLM impart its signatures on the basalts and how would it be possible to distinguish between the SCLM and oceanic mantle? Combining Os isotopes with those of other elements might help to substantiate the inferred oceanic origin of these Gakkel ridge peridotites7.

Liu and colleagues also raise the question of whether the depleted mantle (commonly referred to as the Depleted MORB Mantle) is the source of MORB, but this is more of a semantic issue. MORB certainly originate from melting of the depleted mantle. Th e question that merits further investigation is whether all parts of the depleted mantle contribute equally to MORB generation.

In this regard, the comparison of the neodymium isotopic composition of abyssal

peridotites and basalts from the southwest Indian ridge8 has shown that the relative contribution of enriched and depleted source components is disproportional to their abundance in the mantle. Th e mineralogical composition of diff erent source components determines their melting characteristics and the extent to which their isotopic signatures are transferred to MORB9. Th e paradigm that the isotopic range observed in partial melts directly refl ects that of their source rocks should therefore be considered obsolete as it is strictly true only for isotopically homogeneous sources.

Th e bottom line of recent isotopic investigations of deep-sea peridotites1,8,10 appears to be that both the extent of heterogeneity and the composition of diff erent components in the mantle could be more extreme than recorded by its melting products (MORB), at both the depleted and enriched end of the spectrum.

References1. Liu, C.-Z., et al. Nature 452, 311–316 (2008).2. Walker, R. J. et al. Geochim. Cosmochim. Ac. 66, 4187–4201 (2002).3. Boyet, M. & Carlson, R. W. Science (2005).4. Tackley, P. J. in Treatise on Geophysics (ed Schubert, G.)

438–505 (Elsevier, Amsterdam, 2007).5. Pearson, D. G. & Nowell, G. M. Phil. Trans. R. Soc. Lond.

360, 2383–2410 (2002).6. Schaefer, B. F., Turner, S., Parkinson, I. J., Rogers, N.

& Hawkesworth, C. J. Nature 420, 304–307 (2002).7. Bizimis, M., Griselin, M., Lassiter, J. C., Salters, V. J. M. &

Sen, G. Earth Planet. Sci. Lett. 257, 259–273 (2007).8. Salters, V. J. M. & Dick, H. J. B. Nature 418, 68–72 (2002).9. Stracke, A. et al. Geochem. Geophys. Geosyst.

4, doi:10.1029/2001GC000201 (2003).10. Harvey, J. et al. Earth Planet. Sci. Lett. 244, 606–621 (2006).

Earle R. Williamsis at the Parsons Laboratory, Massachusetts Institute of technology, Cambridge, Massachusetts 02139, USA.

e-mail: earlew@ll.mit.edu

L ightning is chaotic, meandering and fi tful. Lightning paths are kinked and tortuous, and depart substantially

from the smooth lines of textbook-drawn electric fi elds. Th e late atmospheric electrician Bernard Vonnegut once remarked: “What theoretician would have

predicted lightning?” Th e unpredictable nature of sometimes lethal lightning has long posed a hazard to mankind, and more recently to electrical and electronic systems in the technologically developed world. Th e bolt-from-the-blue and the gigantic jet are two forms of lightning previously judged to be exceptional in form and rare in occurrence. On page 233 of this issue, Krehbiel et al.1 show that such lightning fl ashes are more frequent and well behaved than we thought.

Th is advance in understanding lightning has been enabled by preceding developments. Consistent with the bipolar nature of electric charge, lightning fl ashes at full extent are double-ended, much as botanical trees have a root system in the earth, a trunk in the middle and the branches above. Th e simplest electrostatic representation of a thundercloud is a balanced vertical dipole. Th is structure clearly conserves electric charge but it is too simple to account for more than the most common lightning discharge, the intracloud fl ash.

Electrical discharges from thunderstorms include bolts-from-the-blue, blue jets and gigantic jets along with the more common intracloud and cloud-to-ground lightning. All these phenomena can be understood in a single framework.

ATMOSPHERIC SCIENCE

Predictable lightning paths?

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Th e electrical structure of typical thunderstorms that allow fl ashes to escape from the cloud is a tripole with a negative charge centre at mid-cloud level and two positive charge centres above and below2 (Fig. 1a). A negative screening layer forms on the upper cloud boundary as negative ions in the conducting atmosphere outside the cloud are attracted to the upper positive charge. In his 1956 treatise on thunderstorms3, C. T. R. Wilson proposed a dynamic charge imbalance in which a lightning discharge from the negative middle pole to the ground causes an increase in electric fi eld above the upper positive pole (Fig. 1b). Ultimately, this became Wilson’s prediction for sprites, weakly luminous electrical discharges in the mesosphere initiated by energetic lightning in the troposphere, which was established in observations some 70 years later, when measurement techniques had suitably progressed.

Lightning within the cloud is obscured by scattered light but this is not a problem at radio wavelengths. The use of radiofrequency measurements in the very high frequency range therefore makes it possible to map the entire double-ended structure of individual lightning flashes in space and time with remarkable resolution and continuity. Early analyses by hand methods4 required months of work to produce maps for individual flashes, but today, aided by speedy computers and GPS satellite technology for accurate timing, flash maps can be produced in seconds5. This allows the systematic study of large numbers of flashes. It is now recognized that the very high frequency radio waves from the negative end of lightning are substantially stronger than from the positive end, thereby allowing a remote identification of the polarity of flashes.

One recurrent lightning type on the grand scale of the thundercloud that was detected with such radiofrequency observations is the ‘bolt-from-the-blue’ (Fig. 1c). Its positive end lies in the core of the storm but its negative end progresses out of the top of the cloud and subsequently all the way to the ground.

Observations of thunderclouds with sensitive high-speed video cameras have also revealed a host of new electrical discharge phenomena that are so dim and fl eeting they have escaped detection by the naked eye. Th e important new discoveries by video for the study at hand pertain to blue jets (Fig. 1d)6, which extend upward from the tops of active thunderstorms, and to gigantic jets (Fig. 1e) over the ocean7,8, which propagate from the tops of thunderclouds to altitudes approaching 90 km.

Krehbiel and colleagues1 neatly place all these exceptional lightning flashes in the electrostatic context of the thunderstorm via skilful merging of observations with models. The Wilson prediction is essentially the mechanism for the formation of blue jets: these discharges are initiated between the upper positive charge and the negative screening layer at the cloud top. The authors argue plausibly that positive blue jets are scarce in the radiofrequency observations because this end of the lightning ‘tree’ is electromagnetically quiet. However, this leaves open the question of why the inferred lower negative ends are not readily detected in the radiofrequency measurements.

How does lightning escape from the thundercloud? According to an idea planted 50 years ago9, the lower positive charge works as a kind of escape valve for the high negative charge at mid-cloud level, and teases lightning initiated within the cloud downward. Because of the difference in electric potential, the lightning flash then overshoots the lower positive charge on its way to the ground. According to Krehbiel and colleagues1, the upper positive charge acts in a similar fashion, as a valve for the upward propagating bolts-from-the-blue and gigantic jets, with some modulation by the negative screening layer at the upper cloud boundary.

The electric potential of the main negative charge, typically 107 volts or

more, is huge in comparison with the potential difference between the Earth and the upper atmosphere (2.5 × 105 volts). This relatively small potential difference is not sufficient to sway the much more forceful lightning flash either up into the upper atmosphere or down to Earth. Therefore, whether the lightning goes up to become a gigantic jet or down to become a bolt-from-the-blue when it leaves the cloud top is essentially based on the toss of a coin.

Any predictability of the ultimate fate of an upward electrical discharge — gigantic jet versus bolt-from-the-blue — suggested by Krehbiel and co-workers rests in the exact dispositions of the local screening layers, which are more readily treated with models than they are available from observations. Nevertheless, their work provides an important heads-up on lightning’s next move.

References1. Krehbiel, P. R. et al. Nature Geosci. 1, 233–237 (2008).2. Simpson, G. C. & F. J. Scrase Proc. R. Soc. A

161, 309–352 (1937).3. Wilson, C. T. R. Proc. R. Soc. Lond. A

236, 297–317 (1956).4. Proctor, D. E. J. Geophys. Res. 76, 1478–1489 (1971).5. Krehbiel, P. R. et al. EOS 81, 21 (2000).6. Wescott, E. M., Sentman, D., Osborne, D., Hampton, D.

& Heavner, M. Geophys. Res. Lett. 22, 1209–1212 (1995).

7. Pasko, V. P., Stanley, M. A., Mathews, J. D., Inan, U. S. & Wood, T. G. Nature 416, 152–154 (2002).

8. Su, H. T. et al. Nature 423, 974–976 (2003).9. Clarence, N. D. & Malan, D. J. Q. J. R. Meteorol. Soc.

83, 161–172 (1957).

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Figure 1 Flashes of lightning. a, The electric tripole structure of a thunderstorm with a negative electric screening layer at the top can produce a variety of lightning discharges. b, Wilson3 suggested in 1956 that a cloud-to-ground discharge induces an upward discharge above the upper positive charge, thus predicting sprites. c–e, Krehbiel and colleagues1 place bolts-from-the-blue (c), blue jets (d) and gigantic jets (e) in one single framework.